Hydrocarbon distillates vary widely in content. Some may contain a low percentage of non-metallic impurities and a high percentage of aromatic compounds, while others contain a low percentage of aromatics and a high percentage of non-metallic impurities such as nitrogen, sulfur and oxygen. Some distillates may contain varying percentages of both. These are often organic hydrocarbon compounds which contain these impurities as heteroatoms. Nonmetallic impurities adversely affect catalytic hydrocarbon processes such as hydrotreating by poisoning the catalyst. Distillates are hydrotreated for a variety of reasons. Hydrotreating maybe used to remove aromatics and other impurities. Environmental regulations in some areas of the United States currently require the aromatic content of diesel to be no more than 2 wt. %, preferably no more than 10 wt %.
Other distillates, such as those used for the production of kerosene jet fuel, are treated for the removal of heteroatoms. The same catalyst may be used for both aromatic removal and heteroatom removal, although in the past hydrotreating processes employing different feed compositions have been carried out in different reactors.
The concept of the instant invention involves switching back and forth, using the same catalyst or catalyst system, between low heteroatom content feeds, producing low aromatic diesel, and high heteroatom content feeds, producing jet fuel. By using the same catalyst batch in the same reaction zone, the refiner achieves desirable flexibility with reasonable cycle lengths.
The production of low aromatic diesel fuels is becoming increasingly important in both the United States and Europe. Catalysts impregnated with noble metals have been shown to be effective in saturating aromatic compounds. It has been observed (G. L. B. Thielemans, "Manufacturing of low aromatic diesel fuel," 1993 European Oil Refining Conference, 21-22 June 1993, Sitges, Barcelona) that the reduction of aromatic content via the hydrogenation process has a positive effect on all key fuel properties, although at high costs. Key fuel properties include cetane number and cetane index, as well as API gravity. Distillate aromatic content is inversely related to cetane number, the accepted measure of diesel fuel quality. Cetane number and cetane (or diesel) index are both measures of the ignition quality of diesel fuels. Diesel fuels have a minimum cetane number of 45.
Cetane number, which is directly related to ignition quality is highly dependent on the paraffinicity of molecular structures, whether they are straight-chain or alkyl attachments to rings. A distillate stream which comprises mostly aromatic rings with few or no alkyl-side chains generally is of low cetane quality while a highly paraffinic stream is generally of high cetane quality. Dearomatization of refinery distillate streams can increase the volume yield of distillate products. Aromatic distillate components are generally lower in gravity than their similarly boiling paraffinic counterparts. Saturation of aromatic rings can convert these lower API gravity aromatic components to higher API gravity saturated components and expand the volume yield of distillate product.
Hydroprocessing plants are expensive to construct as well as to operate, therefore those that are constructed must be designed for operation that is as efficient as possible.
Aromatic saturation activity is poisoned by even low heteroatom content feedstocks. Consequently, only very low heteroatom content feeds are traditionally fed to catalysts which saturate aromatics. Catalysts impregnated with noble metals can also remove heteroatoms, however. Low sulfur and nitrogen fuels can be produced from feedstocks which contain relatively high concentrations of heteroatoms, such as straight-run kerosene. Prior attempts have been made to deal with problems associated with the poisoning of hydroprocessing catalysts impregnated with metals and the subsequent decrease in their activity. U.S. Pat. No. 3,368,965 (Schuman) discloses the wetting of fresh catalyst by clean oil (non-aromatic oil of low coking tendency). The catalyst can then be heated along with regular heavy charge oil and hydrogen without forming much coke on its surface. The activity of the catalyst is thus improved. Although two different oils are being applied to the catalyst in this invention, only one is intended for conversion to product. One oil is applied to the catalyst only in a small amount as a wetting agent. Furthermore, these oils are not used alternately, in a swing fashion, as in the instant invention.
U.S. Pat. No. 3,900,388 (Hilfman) teaches the removal of aromatic impurities from paraffinic chargestocks. A catalyst which is surface-impregnated with Group VIII metals is contacted with the chargestock in an atmosphere comprising hydrogen. This chargestock has a very low aromatic content, i.e. 1.5 wt %, as opposed to the hydrocracker splitter bottoms or light gas oils of the instant invention, which contain over 30 wt % aromatic compounds. Unlike the present invention, removal of aromatics using the invention of this patent does not result in a higher volume of product.
U.S. Pat. No. 5,152,885 (Singhal et al.) discloses a hydrotreating process employing catalysts impregnated with noble metals and a ligand such as dithiocarbamate. In this process, heteroatoms may be removed from a variety of feeds, including distillates. There is no teaching of aromatics removal, however, or of switching different feeds over the same catalyst in order to restore catalyst activity, as in the instant invention.
U.S. Pat. No. 5,147,526 (Kukes et al.) and U.S. Pat. No. 5,151,172 (Kukes et al.) disclose a process for the hydrogenation of distillates employing the noble metals platinum and palladium impregnated on zeolite Y. Distillates having a high aromatic concentration as well as a high concentration of heteroatoms (from 20 to 60 wt % aromatics, from about 10 ppm to about 0.9 wt % elemental sulfur, and from about 5 ppm to about 900 ppm nitrogen) may be treated in this process. There is no teaching, however, of alternating feeds of varying compositions over the same catalyst, as in the instant invention. There is also no teaching of the use of other catalysts in combination with noble metal catalysts, as in the instant invention.